The present invention relates to an amplifier for analog signal samples, for use for example in the digitising to a high degree of precision of analog signals in the form of samples of such signals with the object of obtaining digital signals with a floating decimal point.
The progress made by the use of digital information, particularly with regard to its treatment, is explained by a general tendency to code, in digital form, analog signals supplied by numerous sources of very different kinds.
Generally the digitising of an analog signal consists in taking samples, measuring their amplitude and possibly their sign, the result of this measurement being expressed in the form of a coded digital signal.
When the dynamic range of the analog signals is increased, their digitising requires a large number of binary positions. This solution is therefore shown to be complicated and is difficult to put into practice, particularly if it is desired to maintain the same relative precision throughout the dynamic range.
For this purpose it has been proposed initially to amplify each sample with an adjustable binary gain so that the amplitude of each sample can be brought in the lower neighbourhood of a fixed value, known as the full-scale signal, and to carry out an analog-to-digital conversion of the thus-amplified sample with this full-scale signal as a measurement standard.
Digital signals with floating decimal point comprise two parts, a characteristic and a mantissa, each containing elementary binary information or bits.
According to this principle, a device has also been proposed capable of both amplifying samples of analog signals with automatic gain control in discrete values and effecting analog-to-digital conversion of each amplified sample. This device was the subject of a French patent application No. 72.00799filed on Jan. 11, 1972 by Societe d'Etudes, Recherches et Constructions Electroniques S E R C E L relating to an "Amplifier for analog signal samples with automatic gain control, and a digitising circuit with floating decimal point for such samples" (U.S. patent application Ser. No. 322,333, filed Jan. 10, 1973 now U.S. Pat. No. 3,936,819.)
For amplification with automatic gain control in this device, the sample circulates in an amplification and delay memory circuit with application of a predetermined binary gain in each cycle. The circulation cycles are counted until the amplified sample reaches the lower neighbourhood of a full-scale signal, and this same number of circulations determines the characteristic of the sample.
One of the difficulties with this device arises from the fact that amplification and conversion are both carried out by the same components. In fact, the errors introduced by each component part, for example the offset voltage of the amplifiers, are repeated both in amplification and in conversion, which in the latter gives these errors a cumulative effect which is prejudicial to the accuracy of the conversion.
In particular, the samples of weakest value are those which need the greatest amplification. On these samples the cumulative amplification errors can become inadmissible for the required conversion accuracy.
This difficulty is all the more apparent if consideration is given to the variation, in terms of time, of the characteristics of the circuits, a variation caused in particular by changes in temperature.
The object of the present invention is to improve the amplification and analog-to-digital conversion circuits described in the abovementioned patent application to enable errors produced by the various circuit components to be compensated.